The present invention relates to a flat motor suitable to use in an office automation device such as copying machine.
FIG. 4 shows an example of a conventional flat motor in partial cross section. In FIG. 4, the flat motor comprises a rear bracket 1 of magnetic material, a permanent magnet 2 secured to a side surface of the rear bracket 1, a front bracket 3 of magnetic material fixed to the rear bracket 1 with a gap therebetween, an armature 4 including a coil 5 disposed between the permanent magnet 2 and the front bracket 3, a commutator 6 connected to the coil 5 and coupled to the armature 4, a rotary shaft 7 supporting the armature 4 and the commutator 6, and bearings 8 and 9 for rotatably supporting the shaft 7. The motor further includes an adapter 10 secured to the rear bracket 1 by bolts which are not shown, a rotation detector 11 constituting a frequency generator, and a brush holder 12 housing brushes 13 and springs 14 for biasing the brushes 13 toward the commutator 6. The rotation detector 11 includes a bobbin 11a on which a coil 11b is wound, a permanent magnet 11c, a stator 11d having teeth formed on an inner periphery thereof, and a rotor 11e fixed to a boss 11f of non-magnetic material and facing the stator 11d. The boss 11f is mounted on the rotary shaft 7 and coupled to the commutator 6 by pins 11g. A cover 11h of magnetic material forms a detector housing.
When a voltage is applied through a feeding circuit (not shown) to the brushes 13, the rotary shaft 7 rotates according to a relation between a current flowing through the coil 5 and magnetic flux of the permanent magnet 2, which is defined by Flemming's law. The rotation of the shaft 7 is controlled by an output of the rotation detector 11. That is, flux from the magnet 11c passes through a magnetic circuit composed of the cover 11h, the rotor 11e and the stator 11d. When the rotor 11e rotates, a voltage pulse corresponding to a variation of magnetic flux which is produced by a variation of gap caused by the teeth of the rotor 11e facing the stator 11d is induced in the coil 11b. The rotation of the motor is controlled on the basis of the number of voltage pulses.
Magnetic flux from the magnet 11c is blocked by the non-magnetic boss 11f provided in an intermediate position of the magnetic circuit to prevent it from leaking through the bearing 9 and the rotary shaft 7 to the rotor 11e, otherwise, the magnet 11c must be large enough to compensate for flux leakage. The boss 11f is usually of stainless steel or brass which is relatively expensive. Further, in order to rotate the boss 11f with the rotary shaft 7, relative rotation between the boss 11f and the shaft 7 is prevented by inserting the pins 11g pressure-inserted into the boss 11f into the commutator 6. Since the pins 11g are small, their manufacture is difficult and their pressure-insertion into the boss 11f is troublesome. In addition, due to the presence of the boss 11f, the axial dimension of the flat motor tends to increase.